Color separation scanners are used to scan original artwork and separate the image into individual color components, i.e. blue, green and red wavebands. The resulting data for the blue, green and red wavebands is most commonly used to produce monochrome halftone separation images on separation films. The monochrome separation films are subsequently used to make printing plates, usually for four color printing with traditional yellow, magenta, cyan and black process inks. Of course, continuous and halftone separation films may be made and used in other processes.
In particular, the present invention pertains to color scanners of the type generally disclosed in United Kingdom patent number 1,600,005 issued to Neilson and Pickering entitled "Improvements In Or Relating To Electro-Optical Scanning". In the scanner there disclosed, reflective or transparent artwork containing an image to be analyzed is mounted on a rotating analyzing drum. An image point on the original artwork is illuminated with focused light from a light source and analyzed as the analyzing drum rotates at high speed with the scanning head advancing transversly across the surface of the drum at a slower speed. Each image point is scanned three times during successive rotations of the drum to obtain density data for the three separate wavelength band components, i.e. blue, green and red, necessary to make yellow, magenta, cyan and black separation films.
The foregoing type of scanner, wherein three successive drum rotations are used to detect three waveband components of an image point, advantageously permits use of a single detector to sense all wavebands. This eliminates the need for color drift compensation between detectors and reduces scanner cost.
Theoretically, three rotations of the analyzing drum are completed prior to advancing the scanning head so that blue, green and red density data is collected from identical points on the original image. It has been found, however, that the simultaneous rotation of the analyzing drum and transverse movement of the scanning head result in misalignment of the blue, green and red images detected for any given point on the original image. In other words, by the time each successive drum rotation is complete the scanning head has advanced slightly, e.g. one third of a pixel, in the longitudinal direction along the drum. Consequently, the blue, green and red images are not in accurate alignment for each image point, and the resulting blue, green and red waveband density data do not correspond precisely for each image point. This causes, among other things, undesirable color fringing at detail edges and increased signal noise.
Therefore, it is one object of the present invention to provide a color scanner wherein all waveband images corresponding to a given image pixel are in optical alignment.
It is a further object of the present invention to provide a scanning color head for a color scanner having a rotating drum, wherein all waveband images corresponding to a given image pixel are in optical alignment.
Another object of the invention is to provide a color scanner that is not susceptible to color fringe effects.
Another object of the invention is to provide a color scanner with improved signal noise.
These and other highly desirable and unusual results are accomplished by the present invention in an economical, reliable and compact color scanner optical system.
Objects and advantages of the invention are set forth herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of instrumentalities and combinations pointed out in the appended claims.
The invention consists of the novel parts, constructions, arrangements, combinations, steps and improvements herein shown and described.